Director Maya Forbes' autobiographical "Infinitely Polar Bear" benefits, to no small degree, from being based on the facts of her childhood. The premise -- a father who's "manic-depressive" (as they used to say) and raising two spirited school-age daughters -- sounds a little like a quirky sitcom, maybe one produced by Netflix. Tarpits of treacle and swamps of sentiment seem to await anyone reckless enough to drive such a vehicle as far as a script meeting. Yet it's largely true. And it rings that way.

The film is enormously charming and funny, thanks to the centerpiece performance by Mark Ruffalo. He brings to the character of Cam Stuart (based on the director's father, Donald Cameron Forbes) a mix of the patrician and the unpredictable. He may drink, both from the scotch bottle and the fount of instability, but his little finger will always be extended. His minor affectations don't diminish Cam's poignancy, or that of his daughters, Faith and Amelia (Ashley Aufderheide and the Forbes' daughter, Imogene Wolodarsky). Nor do they distract from the beyond-long-suffering nature of the girl's mother, Maggie (Zoe Saldana), who is forced to leave the girls behind when she goes off to get her master's degree from Columbia University. The result is authenticity, arrived at through eccentricity.

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Because his family won't give him any money, and Maggie is the sole breadwinner, Cam has to try to pull it together during his 18 months alone with the girls, who possess a fine, balanced mix of love for their father and resentment at his condition; embarrassment at the squalor they usually live in; and bewilderment at living in dirt-cheap housing even though they belong to a filthy rich, Boston Brahmin family (John Kerry is among Forbes' relations).

"Infinitely Polar Bear" -- a title based on a kid-ification of "bipolar" -- doesn't dwell much on the darker aspects of mental illness, nor does it run from the difficulties, which, naturally, provide the plotline. But it's a story about real love and a real family, and by definition comes equipped with peculiarities. In this case, they're just a little more peculiar, and dramatic, and certainly compelling.